1. Field of the Invention
The invention relates to a device for displaying television pictures by means of a color picture tube whose neck accommodates an electron gun system for emitting to a dislay screen three electron beams, a central beam which coincides at least substantially with the axis of the picture tube and two outer beams located on either side thereof, such device including a deflection unit which is secured coaxially around the picture tube, which deflection unit comprises a system of line deflection coils which when energized deflects the electron beam in a first direction and a system of field deflection coils which when energized deflects the electron beam in a direction at right angles to the first direction, said system of field deflection coils comprising two field deflection coils located diametrically with respect to each other, each field deflection coil comprising a plurality of conductors extending in the longitudinal direction of the deflection unit.
2. Description of the Related Art
In color picture tubes of the in-line type the electron gun system is adapted to generate three coplanar electron beams which converge on the display screen. The deflection unit placed around the picture tube for deflecting the electron beams is used to deflect the electron beams in one or in the other direction of their normal non-deflected straight path so that the beams impinge upon selected dots of the display screen to provide visual indications thereon. By varying the magnetic deflection fields in a suitable manner, the electron beams can be moved upwards or downwards and to the left or to the right across the (vertically placed) display screen. By simultaneously varying the intensity of the beams a visual presentation of information or an image can be formed on the display screen. The deflection unit secured around the neck portion of the picture tube comprises two systems of deflection coils to enable the electron beams to be deflected in two directions at right angles with respect to each other. Each system comprises two coils placed on sides facing each other of the neck of the tube, the systems being displaced with respect to each other around the neck of the tube over an angle of 90.degree.. Upon energizing the two systems of deflection coils produce orthogonal deflection fields.
The fields are essentially at right angles to the path of the non-deflected electron beams. A cylindrical core of a magnetizable material, which may tightly enclose the systems of deflection coils if they are both of the saddle-type, is generally used to concentrate the deflection fields and to increase the flux density in the deflection region.
Field deflection coils of the saddle-type are self-supporting coils comprising a plurality of conductors which are wound to form longitudinal first and second side packets, an arc-shaped first end segment and an arc-shaped second end segment together defining a window aperture. In such coils the rear end segments (on the side of the gun) may be flared with respect to the profile of the picture tube (the original type of saddle coil) or they may be arranged flat against the tube wall (in this type of saddle coil the rear end segment follows, as it were, the tube profile). Alternatively, the field deflection coils may be of the type which is toroidally wound on an annular core.
After mounting a deflection unit provided with field deflection coils and line deflection coils on the picture tube for which it is intended, a convergence error is often found to occur during operation, which error becomes manifest as an y-error at the ends of the picture axis. This error is referred to as a picture balance error when the raster written by the one outer beam is larger in the vertical direction than the raster written by the other outer beam. This error is referred to as picture twist error when the raster written by the one outer beam lies above (or below) the raster written by the other outer beam.